Propeller with kort nozzle

ABSTRACT

A propeller assembly including a Kort nozzle is formed of a plurality of propeller blades mounted for rotation on a shaft. The Kort nozzle circumferentially encloses the outer edge of the propeller blades and has an outwardly extending annular groove formed in its inner surface arranged opposite the outer edges of the blades. Secured to the outer edges of the blades for rotation therewith, is an annular member which forms a closure for the opening to the annular groove in the nozzle. The annular member is positioned in combination with the inner surface of the Kort nozzle to form a pair of narrow openings to the annular groove spaced apart in the propulsion direction of the propeller assembly.

United States Patent Taniguchi et a]. [451 July 18, 1972 1 PROPELLERWITH KORT NOZZLE 2,393,113 4/1946 Parrish ..4 5/79 [72] Inventors: KlmTammie; No Min; 3,314,651 4/1967 Beale ..15/l72 3 Kim", FOREIGN PATENTS0R APPLlCATiONS Nagasaki, Japan 226 504 2/1925 Great Britain ..415/172[73] Assrgnee: Mitsubishi Ink 0 Kabuslflld Kahha Tokyo Japan 477,3736/1925 Germany... 486,923 1 1/1929 Germany [22] Filed: March 31, 1970131,435 4/1929 Germany ..172/ [211 App! No; 24513 PrimaryExaminer-Douglas Hart Attorney-McGlew and Toren [30] Foreign ApplicationPriority Data April 8, 1969 Japan... ...44/26586 [57] ABSTRACT June 30,1969 Japan ..44/51085 A ller mbl including a Kort nozzle is formed of aplurality of propeller blades mounted for rotation on a shah. [52] U.S.Cl ..4l$/l72, 60/221 Th: Km nqzzle circumferentially cncloges thg uteredge of [5 Int. the ro eller and has an outwardly extending annular 0f..60/220, l 2, A, gr v f r ed i inner surface arranged opposite theouter 1 14; 415/171 172 edges of the blades. Secured to the outer edgesof the blades 56 f for rotation therewith, is an annular member whichforms a 1 Re mam Cited closure for the opening to the annular groove inthe nozzle. UNITED STATES PATENTS :I'he annular member is positioned incombination with the inner surface of the Kort nozzle to form a pair ofnarrow 3,389,558 6/1968 o ening the annular groove spaced apart in thepropulsion 3,1 l I Jerger l X di -cction cf the propeller 1,820,7258/1931 Bailey ..415/79 2,030,993 2/1936 Langenkamp et a1 ..415/172 X 3Claims, 10 Drawing Figures PROPELLER Wl'l'l-I KORT NOZZLE BRIEFEXPLANATION OF THE DRAININGS FIG. 1 is a diagrammatical fragmentarycross-sectional view of a conventional propeller with Kort nozzle,showing the positional relation between the Kort nozzle and the outerend of the propeller blade; FIGS. 2 and 3 are view of an embodiment ofthe present invention, in which FIG. 2 is a fragmentary verticalcross-sectional view showing the positional relation between a Kortnozzle and the outer end of a propeller blade and FIG. 3 is across-sectional view taken on the line Ill-Ill of FIG. 2; FIGS. 4 to 7inclusive are fragmentary cross-sectional views of other embodiments ofthe invention, respectively; FIGS. 8(a) and 8(b) are a set of views forexplaining the secondary flow of a swirling fluid formed between theKort noale and the annular member at the outer edge of the propellerblade; FIG. 9 is a graph showing the turbulent flow drag coefficient vs.Reynolds number curve obtained by a calculation according to Wendtsformula; and FIG. I is a graph showing the experimental values obtainedby Wendt.

DETAILED DESCRIPTION OF INVENTION The present invention relates toimprovements in the Kort nonle-incorporating propellers.

In general, a propeller with Kort nozzle provided therein produces anextremely large propulsion as compared with a propeller not providedwith such a noule. This is attributed to the following three functionsof the Kort nozzle: namely,

1. function to equalize the entering fluid;

2. function to mitigate the eddy flow occuring at the outer and end ofthe propeller blade by the fluid passage surface of the Kort noule; and

3. function to prevent contraction of the slip stream.

Owing to such functions, the pressure applied to the outside surface ofthe Kort nozzle becomes greater than that applied to the inside surfacethereof and such pressure differential generates a propulsive force inthe entire Kort nozzle, which occasionally reaches from 40 to 50 percentof the entire propulsive force of the propeller, as is well known. Theapplication of the Kort nozzle-incorporating propeller to a tugboat,etc., brings about such and amazing advantage that a 50 percent increasecan be obtained in the shore towing force, i.e. in the death pull, and a30 percent increase can be obtained even in the self-propulsion. On theother hand, the conventional propeller provided with Kort nozzle had thefollowing disadvantage. Namely, in the conventional propeller as shownin FIG. 1, a gap C exists between the Kort nozzle 01 and the outer edgeof the propeller blade 02, and this gap is considerably large because itis usually designed to be about 1/100 of the radius at the throat of theKort nozzle. Therefore, drifting wood or other solid substances in thewater are frequently caught in the gap, causing breakage of thepropeller blade. This not only lowers the working efficiency of theassociated ship but also adds to the cost of repair, with the resultthat the above-mentioned merit in performance is offset. Under thecircumstances, various countermeasures have been proposed and put inpractice for the purpose of obviating the disadvantage. Namely, it hasbeen proposed to exchange a broken propeller blade with a sparepropeller blade, to alleviate the damage to the propeller blade byfitting a flexible material in a portion of the fluid contacting surfaceof the Kort nozzle or to attach a lattice-shaped guard to the frontsurface of the Kort noule to prevent intrusion of solid substancespresent in the water. However, any one of these measures is no more thana passive method conceived with a view to alleviating the damage to thepropeller blade to the possible extent on the concept that theoccurrence of such trouble is inevitable, and it cannot be said thatthey are the measures which fundamentally solve the problem.

The present invention has been achieved in the light of such adisadvantage of the conventional Kort nozzle-incorporating propeller asdescribed above. Namely, it is an object of the present invention toprovide a technical means which will positively eliminate theabove-described trouble or, in other words, to construct a Kortnozzle-incorporating propeller which will not permit solid substances inthe water to be caught between the fluid contacting surface of the Kortnozzle and the outer edge of the propeller blade. It is also an objectof the invention to minimize the power loss of a propeller driving primemover, while simultaneously attaining the object set forth above.

According to one aspect of the present invention, there is provided apropeller assembly which comprises propeller blades, a Kort nozzlehaving an annular groove formed circumferentially in the inner surfacethereof confronting the outer edges of said propeller blades, and anannular member fitted to the entire circumference of the outer edges ofthe propeller blades for rotation with said propeller blades andsubstantially covering said annular groove of the Kort nozzle, thesurface of said annular member facing the fluid flow passageconstituting a part of the inner surface of said Kort nozzle, wherebyintrusion into the gap between the Kort nozzle and the outer edge of thepropeller blade of the solid substances in the water can be prevented.According to another aspect of the invention there is provided apropeller assembly of the character described, wherein the shapes anddimensions of said annular groove and said annular member are sosuitably selected as to decrease to the extent possible the fluidfriction loss due to rotation of the annular member and thereby todecrease the power loss of a propeller driving prime mover to the extentpossible.

The present invention will be described more practically hereinafterwith reference to the embodiments shown in the drawings.

FIGS. 2 and 3 show an embodiment of the present inven tion. Referring toFIGS. 2 and 3, a suitable number of propeller blades 1 (the embodimentshown comprises four of such propeller blade) are integrally connectedat their roots to a propeller boss II] which is secured to a propellershaft 9 by means ofa key 11. An annular member 2 is fixed to the outeredges of the propeller blades I along the entire circumference thereof,and has a front fin 3 and a rear fin 4. A Kort noule 5 is providedcircumferentially of the assembly of said propeller blades and saidannular member and has an annular groove 6 formed in a portion of theinner surface thereof in confronting relation with the outer edges ofthe propeller blades 1. More specifically, the annular groove 6 isformed in a suitable width b and depth d rearwardly from the terminalend of the convergence of the fluid passage defined by the Kort nozzle 5or the point from which said fluid passage expands outwardly forwardlyor, in other words, from the leading end portion of the minimumcross-section of said Kort nozzle 5 which has previously beendetermined, and an annular chamber is defined by said annular groove 6and the outer surface 20 of the annular member 2. Therefore, the annulargroove 6 is substantially covered by the annular member 2 and the innersurface 217 of said annular member 2 constitutes a part of the innersurface of the Kort nozzle 5, defining the fluid passage. Gaps C and Care formed between the leading and trailing edges of the annular groove6 in the Kort nozzle 5 and the front and rear fins 3 and 4 of theannular member 2 respectively, and the sizes of these gaps should be assmall as possible and is preferably not larger than 11100 of the throatradius of the Kort nozzle 5, provided that smooth rotation of thepropeller blades 1 is ensured. At fin 50 may be provided extendingrearwardly from the front lower corner of the annular groove 6 andtaking a part of the front fin 3 of the annular member 2, to preventintrusion of solid substances into the gap. The Kort home of thestructure described above is supported by a bracket 7 firstly fixed to aportion of the hull 8.

In the propeller assembly shown, when the propeller blades 1 are rotatedby driving the propeller shah 9, the hull is propelled by the propulsionof said propeller blades I and water flows through the passage definedby the Kort nozzle as indicated by the arrows. In this case, the annularmember 2 fixed to the outer edges of the propeller blades l rotatestogether with said propeller blades, while covering the annular groove 6of the Kort node 5. Further, since the inner surface 2b of the annularmember 2 constitutes a part of the inner surface of the Kort nozzle 5,providing for smooth passage of the fluid through the passage, there isno fear of solid substances being caught in the gap between the outeredge of the propeller blade and the inner surface of the Kort nozzle 5.In addition, the propeller blade 1 is naturally deflected in thedirection of propulsion, incident to rotation of the propeller blade,under a load imposed thereon and hence the gap Q is further reduced,which makes the intrusion of solid substances through the gap C moredifficult. Even if a small solid substance is allowed to enter theannular chamber through the gap C,, it is readily discharged through thegap C as said gap C, remains the same in size and is larger than the gapC, Thus, the annular member 2 is protected against damage.

Because of such construction and function, the propeller assemblydescribed above demonstrates the effects of the Kortnozzle-incorporating propeller as such and has the advantage that it isentirely free of accident caused by solid substances floating in thewater.

Another embodiment of the present invention is shown in FIG. 4. Thisembodiment is different from the preceding embodiment in that the rearhalf portion of the Kort nozzle 5 is removable so that the annularmember 2 fixed to the propeller blades 1 may be received in the annulargroove 6 of the Kort nozzle and further in that the inner surface 26 ofthe annular member is made substantially flush with the inner surface ofthe Kort nozzle 5 to define the flow passage,

Still another embodiment of the invention is shown in FIG. 5. Thisembodiment differs from the first-described embodiment in that theannular member 2 is shaped into a substantially wing-like configurationand its streamlined inner surface 2b of a contour approximating that ofthe streamlined inner surface of the Kort nozzle 5 is made flush withthe inner surface of said Kort nozzle to define the flow passage. In thesecond and third embodiments, the inner surface of the Kort nozzle 5 andthe inner surface 2b of the annular member 2 are substantially flushwith each other to provide a smooth surface of flow passage as statedabove. It will, therefore, be obvious that these embodiments achieve thesame effects as those of the first embodiment, without substantiallyreducing the propulsion of the propeller assembly.

The fourth embodiment of the invention shown in FIG. 6 isdistinguishable from the preceding three embodiments in that thedistance d between the outer surface of the annular member 2 and thebottom surface 60 of the annular groove 6 of the Kort noule 5 is made aslarge as possible within the range tolerated by the structural strengthof the Kort nozzle 5. The filth embodiment of the invention shown inFIG. 7 differs from the respective embodiments described above in thatthe distance d is made large as in the case of the preceding fourthembodiment and also in that the width b of the annular member 2 isreduced and the forward end portion of the inner surface 2b of theannular member 2 is stepped up by a distance 8 relative to the innersurface of the Kort nozzle 5.

Considering the fluid friction loss due to rotation of the annularmembers in the fourth and fifth embodiments, the friction loss of theannular member 6 may be regarded as sort of friction loss of a rotatingcylinder. In obtaining the friction loss of a rotating cylinder, adimensionless number of d d/r, is employed wherein d is the depth of theannular groove 6 and r is the radius of the propeller blade in FIG. 6.It is well known that with regard to the relationship between theturbulent flow drag coefficient f and the Reynolds number Red, Wendtobtained the experimental values indicated by the markings O D and A inFIG. 10 when the values of d* is 0.470, 0.176 and 0.0691 respectively,and that based on the experimental values the relationship between theturbulent flow drag coefficient and the Reynoldsnumber Red* is given bythe formula within the range of l0 Re l0. The values off and d,calculated from the above formula are in such relation mm the formerbecomes smaller as the latter becomes larger, as shown in FIG. 9.Further, although it is not certain from the graph of FIG. [0 whetherthe value of f becomes further smaller or remains substantially the samein the range of l0 Re 10' in the proximity of the normal r.p.m. of theKort nozzle-incorporating propeller, the friction loss due to asymmetrical eddy as shown in FIG. 8(a) (which is produced secondarily bythe centrifugal force created in the fluid by the rotation of theannular member 2 and the boundary of the vertical walls of the annularchamber) is included in the turbulent flow drag coefiicient f and is sosmall as will not pose a practical problem. It will thus be seen that inorder to minimize the friction loss due to rotation of the annularmember 2, it is preferable to make the value ofd as large as possible asin the fourth and fifth embodiments. Now, considering the circulation offluid around the annular member 2 as shown in FIG. 8 (b), the pressurecoefficient d: which is a dimensionless value of pressure rise iswherein u is the peripheral speed of the propeller blade. Therefore, inno case will the velocity Cmb of the circulating fluid exceed the valuewhich is considered to result when the static pressure rise of thepropeller is entirely converted into dynamic pressure, namely (Cmb/u)fi= 0.7. In practice, however, the fluid passing through the propellerflows at a consistent velocity as a whole, as a result of branching,confluence, curving and sudden expansion of cross-sectional area of theflow passage, occurring in the vicinity of the gaps C, and C and it ispossible to consider that Cmb/u 1.0. Nevertheless, the fear of smallsolid substances entering the annular chamber through the gap C at thetrailing end of the annular member 2 cannot definitely be denied,insofar as the above-mentioned circulating flow exists, no matter howminor it may be. Therefore, according to the present invention the rearfin 4 of the annular member 2 is extended slightly rearwardly from therear lower corner of the annular groove 6 of the Kort nonle 5 whilemaintaining the gap C, between it and the inner surface of the Kortnoule S, as shown in FIGS. 2, 5 and 7, whereby the circulating flow canbe minimized and hence the fear of solid substances entering the annularchamber through the gap C can be eliminated, by letting the solidsubstances by carried downstream along with the fluid. Therefore, itwill be obvious that the rear fin 4 is preferably formed as describedabove.

The width b of the annular member 2 should be made as small as possibleas shown in FIGS. 6 and 7, because it has a direct influence on thefriction loss.

In the fifth embodiment shown in FIG, 7, the inner surface 2b of theannular member 2 is stepped up by a distance S relative to the innersurface of the Kort nozzle 5. Therefore, the fluid friction loss due torotation of the annular member can be further decreased.

As will be clearly understood from the foregoing practical descriptionson various embodiments, the present invention brings about theadvantages that the drawbacks of the conventional Kortnoule-incorporating propellers can completely be eliminated and that thepower loss due to rotation of the annular member can be minimized, and,therefore, is of great industrial advantage.

Although the present invention has been described and illustrated withreference to specific embodiments thereof, it should be understood thatthe invention is not restricted only to the embodiments shown but manychanges and modifications are pomible within the scope of the inventiondefined in the appended claims.

What is claimed is:

l. A propeller assembly comprising a rotatable shaft, a plurality ofpropeller blades secured to and disposed in angularly spacedrelationship on said shaft for rotation therewith, said propeller bladesextending outwardly from said shaft, said propeller blades extending inthe propulsion direction of the assembly and having a leading surfacefacing in the propulsion direction and a trailing surface facing in theopposite direction, a Kort nozzle circumferentially enclosing the outeredges of said propeller blades and said Kort nonle extending forwardlyof the leading surface of said propeller blades and rearwardly from thetrailing surface of said propeller blades, the inner surface of saidKort noule intermediate its leading edge and trailing edge forms anannular groove aligned opposite and spaced outwardly from the outeredges of said propeller blades, and an annular member secured to theouter edges of said propeller blades for rotation therewith and disposedopposite said annular groove, the dimension of said annular member inthe propulsion direction being sufficient to form a coveringsubstantially closing the opening to said annular groove and arranged toprovide a narrow circumferentially extending opening to said annulargroove along its leading and trailing edges, the inner surface of saidannular member being arranged in general alignment with the innersurface of said Kort nozzle, said leading edge of said annular membersloping from its inner surface to its outer surface in the propulsiondirection and the juxtaposed surface of said Kort member at the inneredge of said annular groove is closely spaced from and slopes in thesame direction as the leading edge of said annular member forming theopening to the leading end of said annular groove, and the trailing edgeof said annular member extends in the direction opposite to thepropulsion direction so that it is spaced outwardly from and coextendsfor a portion of the inner surface of said Kort nozzle extending fromsaid annular groove toward the trailing edge thereof and provides theopening therebetween for the trailing end of said annular groove.

2. A propeller assembly comprising a rotatable shalt, a plurality ofpropeller blades secured to and disposed in angularly spacedrelationship on said shaft for rotation therewith, said propeller bladesextending outwardly from said shaft, said propeller blades extending inthe propulsion direction of the assembly and having a leading surfacefacing in the propulsion direction and a trailing surface facing in theopposite direction, a Kort noule circumferentially enclosing the outeredges of said propeller blades and said Kort nozzle extending forwardlyof the leading surface of said propeller blades and rearwardly from thetrailing surface of said propeller blades, the inner surface of saidKort nozzle intermediate its leading edge and trailing gdgeformsanannular groove aligned ropposite and spaced outwardly from theouteredges of said propellerblades, and an annular member secured to theouter edges of said propeller blades for rotation therewith and disposedopposite said annular groove, the dimension of said annular member inthe propulsion direction being sufficient to form a coveringsubstantially closing the opening to said annular groove and arranged toprovide a narrow circumferentially extending opening to said annulargroove along its leading and trailing edges, the inner surface of saidannular member being arranged in general alignment with the innersurface of said Kort nozzle, said leading edge of said annular membersloping from its inner surface to its outer surface in the propulsiondirection and the juxtaposed surface of said Kort nozzle at the inneredge of said annular groove is closely spaced from and slopes in thesame direction as the leading edge of said annular member forming theopening to the leading end of said annular groove, and the inner andouter surfaces of said annular member tapered to and meet in an apex atthe trailing edge thereof, and the trailing edge extends in thedirection opposite to the propulsion direction so that it is closelyspaced inwardly from and coextends for a portion of the inner surface ofsaid Kort noule extending from said annular groove toward the trailingedge thereof and provides the opening therebetween from the trailing endof said annular groove.

3. A propeller assembly comprising a rotatable shaft, a plurality ofpropeller blades secured to and disposed in angularly spacedrelationship on said shaft for rotation therewith, said propeller bladesextending outwardly from said shaft, said propeller blades extending inthe propulsion direction of the assembly and having a leading surfacefacing in the propulsion direction and a trailing surface facing in theopposite direction, a Kort nozrle circumferentially enclosing the outeredges of said propeller blades and said Kort nozzle extending forwardlyof the leading surface of said propeller blades and rearwardly from thetrailing surface of said propeller blades, the inner surface of saidKort nozzle intermediate its leading edge and trailing edge forms anannular groove aligned opposite and spaced outwardly from the outeredges of said propeller blades, and an annular member secured to theouter edges of said propeller blades for rotation therewith and disposedopposite said annular groove, the dimension of said annular member inthe propulsion direction being sufficient to form a coveringsubstantially closing the opening to said annular groove and arranged toprovide a narrow circumferentially extending opening to said annulargroove along its leading and trailing edges, the inner surface of saidannular member being arranged in general alignment with the innersurface of said Kort noule, said inner leading edge of said annulargroove is spaced radially inwardly relative to the trailing edge of saidannular groove, the leading edge inner surface of said annular member isspaced closely outwardly from the adjacent inner surface of said Kortnozzle and the trailing edge of the inner surface of said annular memberextends in the direction opposite to the propulsion direction rearwardlybeyond the trailing edge of said annular groove and is spaced closedlyinwardly from the inner surface of the portion of said Kort nozzleextending from said annular groove to the trailing edge thereof so thatthe opening to the trailing end of said annular groove is positionedbetween the outer trailing edge surface of said annular member and thejuxtaposed inner surface of said Kort nozzle extending rearwardly fromthe inner trailing edge of said annular groove.

t i i i i

1. A propeller assembly comprising a rotatable shaft, a plurality ofpropeller blades secured to and disposed in angularly spacedrelationship on said shaft for rotation therewith, said propeller bladesextending outwardly from said shaft, said propeller blades extending inthe propulsion direction of the assembly and having a leading surfacefacing in the propulsion direction and a trailing surface facing in theopposite direction, a Kort nozzle circumferentially enclosing the outeredges of said propeller blades and said Kort nozzle extending forwardlyof the leading surface of said propeller blades and rearwardly from thetrailing surface of said propeller blades, the inner surface of saidKort nozzle intermediate its leading edge and trailing edge forms anannular groove aligned opposite and spaced outwardly from the outeredges of said propeller blades, and an annular member secured to theouter edges of said propeller blades for rotation therewith and disposedopposite said annular groove, the dimension of said annular member inthe propulsion direction being sufficient to form a coveringsubstantially closing the opening to said annular groove and arranged toprovide a narrow circumferentially exTending opening to said annulargroove along its leading and trailing edges, the inner surface of saidannular member being arranged in general alignment with the innersurface of said Kort nozzle, said leading edge of said annular membersloping from its inner surface to its outer surface in the propulsiondirection and the juxtaposed surface of said Kort member at the inneredge of said annular groove is closely spaced from and slopes in thesame direction as the leading edge of said annular member forming theopening to the leading end of said annular groove, and the trailing edgeof said annular member extends in the direction opposite to thepropulsion direction so that it is spaced outwardly from and coextendsfor a portion of the inner surface of said Kort nozzle extending fromsaid annular groove toward the trailing edge thereof and provides theopening therebetween for the trailing end of said annular groove.
 2. Apropeller assembly comprising a rotatable shaft, a plurality ofpropeller blades secured to and disposed in angularly spacedrelationship on said shaft for rotation therewith, said propeller bladesextending outwardly from said shaft, said propeller blades extending inthe propulsion direction of the assembly and having a leading surfacefacing in the propulsion direction and a trailing surface facing in theopposite direction, a Kort nozzle circumferentially enclosing the outeredges of said propeller blades and said Kort nozzle extending forwardlyof the leading surface of said propeller blades and rearwardly from thetrailing surface of said propeller blades, the inner surface of saidKort nozzle intermediate its leading edge and trailing edge forms anannular groove aligned opposite and spaced outwardly from the outeredges of said propeller blades, and an annular member secured to theouter edges of said propeller blades for rotation therewith and disposedopposite said annular groove, the dimension of said annular member inthe propulsion direction being sufficient to form a coveringsubstantially closing the opening to said annular groove and arranged toprovide a narrow circumferentially extending opening to said annulargroove along its leading and trailing edges, the inner surface of saidannular member being arranged in general alignment with the innersurface of said Kort nozzle, said leading edge of said annular membersloping from its inner surface to its outer surface in the propulsiondirection and the juxtaposed surface of said Kort nozzle at the inneredge of said annular groove is closely spaced from and slopes in thesame direction as the leading edge of said annular member forming theopening to the leading end of said annular groove, and the inner andouter surfaces of said annular member tapered to and meet in an apex atthe trailing edge thereof, and the trailing edge extends in thedirection opposite to the propulsion direction so that it is closelyspaced inwardly from and coextends for a portion of the inner surface ofsaid Kort nozzle extending from said annular groove toward the trailingedge thereof and provides the opening therebetween from the trailing endof said annular groove.
 3. A propeller assembly comprising a rotatableshaft, a plurality of propeller blades secured to and disposed inangularly spaced relationship on said shaft for rotation therewith, saidpropeller blades extending outwardly from said shaft, said propellerblades extending in the propulsion direction of the assembly and havinga leading surface facing in the propulsion direction and a trailingsurface facing in the opposite direction, a Kort nozzlecircumferentially enclosing the outer edges of said propeller blades andsaid Kort nozzle extending forwardly of the leading surface of saidpropeller blades and rearwardly from the trailing surface of saidpropeller blades, the inner surface of said Kort nozzle intermediate itsleading edge and trailing edge forms an annular groove aligned oppositeand spaced outwardly from the outer eDges of said propeller blades, andan annular member secured to the outer edges of said propeller bladesfor rotation therewith and disposed opposite said annular groove, thedimension of said annular member in the propulsion direction beingsufficient to form a covering substantially closing the opening to saidannular groove and arranged to provide a narrow circumferentiallyextending opening to said annular groove along its leading and trailingedges, the inner surface of said annular member being arranged ingeneral alignment with the inner surface of said Kort nozzle, said innerleading edge of said annular groove is spaced radially inwardly relativeto the trailing edge of said annular groove, the leading edge innersurface of said annular member is spaced closely outwardly from theadjacent inner surface of said Kort nozzle and the trailing edge of theinner surface of said annular member extends in the direction oppositeto the propulsion direction rearwardly beyond the trailing edge of saidannular groove and is spaced closedly inwardly from the inner surface ofthe portion of said Kort nozzle extending from said annular groove tothe trailing edge thereof so that the opening to the trailing end ofsaid annular groove is positioned between the outer trailing edgesurface of said annular member and the juxtaposed inner surface of saidKort nozzle extending rearwardly from the inner trailing edge of saidannular groove.